Disposable absorbent articles (such as diapers) include typically an absorbent core structure with superabsorbent polymers, typically hydrogel-forming water-swellable polymers (also referred to as absorbent gelling material, AGM, or super-absorbent polymers, SAP's). This polymer material ensures that in use, large amounts of bodily fluids, e.g. urine, can be absorbed by the article and locked away, thus providing low rewet and good skin dryness.
These water-swellable or superabsorbent polymers need to have adequately high sorption capacity, as well as adequately high gel strength. Sorption capacity needs to be sufficiently high to enable the absorbent polymer to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. Together with other properties of the gel, gel strength relates to the tendency of the swollen polymer particles (i.e. gel) to resist deformation under an applied stress in the absorbent article. The gel strength needs to be high enough in the absorbent article so that the particles do not deform too much and thereby fill the capillary void spaces to an unacceptable degree, which would cause so-called gel blocking. This gel-blocking inhibits the rate of fluid uptake and/or the fluid distribution: i.e. once gel-blocking occurs, it can substantially impede the distribution of fluids to relatively dry zones or regions in the absorbent article; then, leakage from the absorbent article can take place well before the superabsorbent polymer particles are fully saturated or before the fluid can diffuse or wick past the “blocking” particles into the rest of the absorbent article. Thus, it is important that the superabsorbent polymers (when incorporated in an absorbent structure or article) maintain a high wet-porosity and have a high resistance against deformation thus yielding high permeability for fluid transport through the swollen gel bed.
Absorbent polymers with relatively high permeability can be made by increasing the level of internal crosslinking or surface crosslinking, which increases the resistance of the swollen gel against deformation by an external pressure (such as the pressure caused by the wearer), but these techniques typically also reduce the absorbent capacity of the gel undesirably.
In addition, there is also a need for superabsorbent polymer particles that have a greater speed of absorption. It has been found that the prior art superabsorbent polymers that may have high gel strength, may often not have a high absorption speed.
In recent years, so-called core-shell superabsorbent polymer particles have been developed that have a superabsorbent polymer core and a shell of elastomeric and/or film-forming polymers, that form an elastomeric shell on the surface of the core, which is elastomeric when dry and when wet, and that reduces deformation of the core-shell particles and that thereby improve the gel strength, whilst having excellent absorbent capacity and speed. For example U.S. Pat. No. 7,049,000 and U.S. Pat. No. 7,270,881 describe such core-shell superabsorbent polymers with an elastic, film-forming thermoplastic polymer shell. However, some of said thermoplastic elastomers may be difficult to process because they may be tacky. Also, some other elastomeric polymers may be hydrophobic.
The inventors have now found alternative coatings for superabsorbent polymer particles, that improve the resistance to deformation (when swollen and under pressure) of said particles when wet, resulting in an improved gel strength, whilst providing excellent absorbent capacity and speed of absorption, and liquid retention under pressure. Said coated superabsorbent particles may furthermore be produced by commercially viable processes.